feature: Show all classes in inheritance diagrams, not just the ones that can be resolved in the current project

[pawamoy]

Is your feature request related to a problem? Please describe.

Currently if a class inherits from a class of another package, this other class is not resolved and therefore not shown in the diagram.

Describe the solution you'd like

By loading the right inventory, we know where the docs for this class are, so we should still display it. If there's no inventory or no docs for this class, that's fine, we can show a node without a link.